InGaN/GaN light-emitting diode with a polarization tunnel junctionZi-Hui Zhang, Swee Tiam Tan, Zabu Kyaw, Yun Ji, Wei Liu et al. Citation: Appl. Phys. Lett. 102, 193508 (2013); doi: 10.1063/1.4806978 View online: http://dx.doi.org/10.1063/1.4806978 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v102/i19 Published by the American Institute of Physics. Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

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InGaN/GaN light-emitting diode with a polarization tunnel junction

Zi-Hui Zhang,1 Swee Tiam Tan,1 Zabu Kyaw,1 Yun Ji,1 Wei Liu,1 Zhengang Ju,1

Namig Hasanov,1 Xiao Wei Sun,1,2,a) and Hilmi Volkan Demir1,3,4,a)

1LUMINOUS! Centre of Excellence for Semiconductor Lighting and Displays, School of Electrical andElectronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore2Department of Electronic and Computer Engineering, South University of Science and Technology of China,Shenzhen, Guangdong 518055, China3School of Physical and Mathematical Sciences, Nanyang Technological University, 50 Nanyang Avenue,639798 Singapore4Department of Electrical and Electronics, Department of Physics, and UNAM-Institute of Material Scienceand Nanotechnology, Bilkent University, TR-06800 Ankara, Turkey

(Received 2 April 2013; accepted 29 April 2013; published online 15 May 2013)

We report InGaN/GaN light-emitting diodes (LED) comprising in situ integrated pþ-GaN/InGaN/

nþ-GaN polarization tunnel junctions. Improved current spreading and carrier tunneling probability

were obtained in the proposed device architecture, leading to the enhanced optical output power

and external quantum efficiency. Compared to the reference InGaN/GaN LEDs using the

conventional pþ/nþ tunnel junction, these devices having the polarization tunnel junction show a

reduced forward bias, which is attributed to the polarization induced electric fields resulting from

the in-plane biaxial compressive strain in the thin InGaN layer sandwiched between the pþ-GaN

and nþ-GaN layers. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4806978]

Significant efforts have been devoted to boosting the

optical output power and enhancing the external quantum

efficiency (EQE) of InGaN/GaN light-emitting diodes

(LEDs).1–3 These approaches include charge seperation

suppression via quantum well engineering,4–6 barrier

engineering,7–12 electron blocking layer (EBL)

engineering,13–16 and novel epitaxy methods for dislocation

density suppression.17,18 Recently, these efforts have also

been extended to improving the current spreading and, thus,

the EQE and output power of InGaN/GaN LEDs.19–21

However, the improved current spreading can be achieved

either by inserting a resistive layer into the p-GaN layer,

or increasing the conductivity of the contact layer for

p-electrode.21 For this purpose, the pþ/nþ tunnel junction has

previously been proposed to enhance the lateral current dis-

tribution in InGaN/GaN LEDs.22–26 In these devices, the

heavy doping in GaN layers induces a strong built-in electric

field, which aligns the conduction band of the nþ-GaN layer

with the valence band of the pþ-GaN layer.27 However, this

tunnel region is a homojunction with no polarization induced

electric fields and yields a low level of tunneling efficiency.

Moreover, the additional voltage consumption in the tunnel

junction significantly increases the forward voltage of the

resulting LED device. Here, different than the previous

reports, to enhance the tunneling efficiency and reduce the

voltage drop across the tunnel junction, we propose and dem-

onstrate the InGaN/GaN LED integrated with a polarization

tunnel junction.

III-nitride epitaxial layers grown along c-orientation are

well known to exhibit strong spontaneous polarization and

piezo-electric polarization,28 which induce positive and nega-

tive sheet charges with relatively high densities at the hetero-

junction interfaces. These charges are able to generate strong

electric field resulting in the band bending, similar to the

ionized dopants in the pþ/nþ homojunctions. Hence, the tunnel-

ing probability can be significantly affected by the strong polar-

ization. The polarization tunneling has been investigated for

both the metal-face (Ga/Al/In-face for cþ growth orientation)

and the nitrogen-face (N-face for c� growth orientation) III-

nitride heterojunctions,29–31 and excellent tunneling probability

was obtained through those polarized junctions. However, to

date, polarization tunneling phenomenon has not been investi-

gated or demonstrated for InGaN/GaN LEDs. Thus, in this

work, to understand the effect of the polarization tunnel junc-

tion on both the current spreading and the carrier tunneling, we

integrated a pþ-GaN/InGaN/nþ-GaN polarization tunnel junc-

tion into the InGaN/GaN LED architecture. In proposed device,

enhanced optical output power and EQE are observed. This is

explained by improved current spreading and increased carrier

tunneling enabled by the polarization tunneling.

For our experiments, three types of InGaN/GaN LED

samples were grown on c-sapphire substrates by our metal-

organic chemical-vapor deposition (MOCVD) system.32 The

growth was initiated on a 30 nm thick GaN nucleation layer.

Then, a 4 lm thick undoped GaN (u-GaN) layer was grown,

followed by a 2 lm Si-doped GaN (n-GaN) layer with a dop-

ing concentration of 5� 1018 cm�3. Subsequently, five peri-

ods of In0.15Ga0.85N/GaN multiple quantum wells (MQWs)

were grown. The thickness of the quantum barriers and quan-

tum wells is 12 and 3 nm, respectively. On top of the

MQWs, a 25 nm Mg-doped-Al0.15Ga0.85 N layer was grown

as the EBL. After that, a 0.2 lm thick Mg-doped GaN

(p-GaN) layer with a hole concentration of 3� 1017 cm�3

was grown. For the Reference Device [see Fig. 1(a)], a

30 nm thick heavily Mg-doped GaN (pþ-GaN) layer was

finally grown as the p-type contact layer. For our epitaxial

wafers, the flow rates of Cp2Mg and TMGa are 1.3 lmol/min

and 22.0 lmol/min, respectively, and the ionization ratio of

Mg dopants at room temperature is 1% in GaN.33 Thus, the

ionized Mg doping concentration in the pþ-GaN layer isa)Electronic addresses: HVDEMIR@ntu.edu.sg and EXWSUN@ntu.edu.sg

0003-6951/2013/102(19)/193508/5/$30.00 VC 2013 AIP Publishing LLC102, 193508-1

APPLIED PHYSICS LETTERS 102, 193508 (2013)

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estimated to be 3� 1019 cm�3. For Device A with the con-

ventional tunnel junction [see Fig. 1(b)], another 30 nm nþ-

GaN layer with the Si doping concentration of 1� 1020

cm�3 was grown on the pþ-GaN layer. In order to reduce the

Mg diffusion, the following nþ-GaN layer was grown under

the same condition as pþ-GaN, except that Cp2Mg was

replaced by SiH4. Meanwhile, considering the compensation

effect of Mg dopants, the nþ-GaN layer was intentionally

doped at a higher level than the pþ-GaN layer. As for Device

B with the polarization tunnel junction [see Fig. 1(c)], a 3 nm

thick undoped In0.15Ga0.85 N layer was sandwiched between

the pþ-GaN layer and nþ-GaN layer. The growth condition

for In0.15Ga0.85 N is the same as that for the quantum well

region.

After growing these described epi-layers in our

MOCVD system, we further fabricated three sets of device-

level samples. During fabrication, the LED mesa with a chip

size of 350� 350 lm2 was patterned by using reactive ion

etch (RIE). Indium tin oxide (ITO) of 200 nm was sputtered

as the transparent current spreading layer. Finally, Ti/Au

(30 nm/150 nm) was deposited by e-beam evaporation serv-

ing as the p-contact and n-contact.

We also performed numerical simulations to understand

the underlying device physics by APSYS,9 which self-

consistently solves the Poisson equation, continuity equation,

and Schr€odinger equation with proper boundary conditions.

The self-consistent six-band k � p theory is used to take

account of the carrier screening effect in InGaN quantum

wells.34 In our simulations, the Auger recombination coeffi-

cient is taken to be 1� 10�30 cm6 s�1.35 The Shockley-Read-

Hall (SRH) lifetime for electron and hole is set to be 43 ns.35

Meanwhile, a 40% of the theoretical polarization induced

sheet charge density is assumed due to the crystal relaxation

through dislocation generation during the growth.28 The

energy band offset ratio of DEc/DEv¼ 70/30 is set in the

InGaN/GaN quantum well regions.36 The other parameters

used in the simulation can be found elsewhere.37

Figs. 2(a) and 2(b) show the experimental and simulated

injection current as a function of the applied bias. The simu-

lated current-voltage characteristics of the studied devices

agree well with the experimental ones. It can be seen from

Figs. 2(a) and 2(b) that the Reference Device and Device A

(with the conventional pþ/nþ tunnel junction) have the low-

est and highest forward voltage, respectively. On the other

hand, the forward voltage is reduced in Device B when the

polarization tunnel junction is used. The improved electrical

performance in Device B compared to Device A is attributed

to the enhanced tunneling probability [Pt as shown in

Eq. (1)] of the carrers in the pþ-GaN/InGaN/nþ-GaN region

Pt � exp �p� m�1=2 � E3=2g

2ffiffiffi2p

e� �h� E

!; (1)

where m* is the effective mass of the carriers in the tunnel

layer and Eg is the energy bandgap of the tunnel region while

E is the electric field, which assists the carrier tunneling.27

According to Eq. (1), the tunnel region with a small

energy bandgap produces a large tunneling probability. Since

the InGaN tunnel junction in Device B has a smaller energy

bandgap compared to the GaN tunnel junction in Device A,

Device B leads to a higher tunneling probability than Device

A.30 Also, the additional polarization induced electric field in

the pþ-GaN/InGaN/nþ-GaN junction further increases the

carrier tunneling probability. Fig. 3 shows the calculated

FIG. 1. Device architectures: (a) Reference Device, (b) Device A with the conventional pþ/nþ tunnel junction, and (c) Device B with the polarization tunnel

junction.

FIG. 2. Injection current versus applied

bias: (a) experiment and (b) simulation.

193508-2 Zhang et al. Appl. Phys. Lett. 102, 193508 (2013)

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electric fields within the tunnel junction for Device A and

Device B. In the pþ-GaN/nþ-GaN junction of Device A,

besides the field produced by the external applied bias, the

additional electric field is generated by the ionized Si donors

in the nþ-GaN and and Mg acceptors in the pþ-GaN layers.

Nevertheless, when the InGaN layer is sandwiched between

the pþ-GaN/nþ-GaN junction in Device B, the polarization

charges will be generated as indicated in Fig. 1(c). The polar-

ization induced electric field in the compressive-strained

InGaN layer is added as the third electric field component in

the tunnel junction. For that, the magnitude of the total elec-

tric field in the tunnel junction for Device B is larger than

that for Device A, as indicated in Fig. 3. The enhanced elec-

tric field, therefore, results in a better carrier tunneling proba-

bility and a reduced voltage drop in the tunnel region for

Device B when compared to Device A.

Figs. 4(a) and 4(b) present the energy band diagrams of

the tunnel junctions for Devices A and B, respectively. We

can see that, for both devices, the conduction band of the

nþ-GaN layer is well aligned with the valance band of the

pþ-GaN layer. Thus, those electrons in the valance band of

the pþ-GaN layer are able to tunnel into the conduction band

of the nþ-GaN layer through the forbidden band. With this,

holes will be generated in the valance band of the pþ-GaN

layer and then injected into the quantum wells for recombi-

nation under the electric field. However, as indicated in

Fig. 3 for Device A, the tunnel region consists of two electric

fields, i.e., the built-in electric field (Ebi) due to the ionized

dopants and the electric field by the external applied bias

(Eext). On the other hand, in Device B, in addition to the Ebi

and Eext, the polarization induced electric field (Espþpz)

increases further the overall magnitude of the total electric

field. The stronger electric field in Device B promotes the

carrier tunneling probability and thus improves the electrical

and optical performance.

It should be noted that Device B features a higher for-

ward voltage compared to the Reference Device, as shown

both in Fig. 2(a) and 2(b). This is mainly because of two rea-

sons: First, it is difficult to grow high quality crystalline and

thick InGaN layer with high indium content on the p-GaN

layer. Second, the crystal relaxation may happen during the

epitaxial process, and therefore, the actual polarization

charge density in the InGaN/GaN hetero-interface could be

smaller than the theoretical value. In our case, we assumed a

40% of the theoretical polarization induced sheet charge den-

sity in our simulation.28 To assist the tunneling process, we

intentionally heavily doped the GaN layers with Mg and Si

in Device B. However, improvement in the electrical proper-

ties using the polarization tunneling without heavily doping

is theoretically possible according to the report by

Schubert.29 The detailed discussion regarding the effect of

InGaN thickness and InN fraction on improving the electri-

cal property can further be found in the supplementary

material.38

Fig. 5 presents the hole concentration across the MQWs

for the three devices. It shows that the Reference Device has

the smallest hole concentration in each quantum well. In

Device A with the pþ/nþ-tunnel junction, the hole concentra-

tion within the MQWs is increased compared to that in the

FIG. 3. Electric field profile computed across the tunnel junction at 4.5 V for

Device A and Device B. The positive direction of the electric field is along

the growth orientation (i.e., [0001]).

FIG. 4. Energy band diagrams of the

tunnel junction for (a) Device A and (b)

Device B.

FIG. 5. Hole concentrations in InGaN/GaN MQWs for the Reference

Device, Device A, and Device B at 50 mA.

193508-3 Zhang et al. Appl. Phys. Lett. 102, 193508 (2013)

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Reference Device, which is due to an improved current

spreading in the nþ-GaN layer of Device A.21 In the case of

Device B with the pþ-GaN/InGaN/nþ-GaN junction, the

highest hole concentration in the MQWs is observed, which

stems from the improved current spreading effect and higher

carrier tunneling probability.

The electroluminescence (EL) spectra measured for the

Reference Device, Device A and Device B are shown in

Figs. 6(a)–6(c), respectively. We can see that the Reference

Device has the lowest EL intensity. With the incorporation

of the pþ/nþ-tunnel junction (Device A), the emission is

improved due to the improved current spreading in the

nþ-GaN layer.21 Meanwhile, the strongest EL emission in-

tensity is obtained from Device B, as shown in Fig. 6(c). The

enhanced optical performance in Device B results from the

improved current spreading effect21 and the enhanced carrier

tunneling probability in the polarization tunnel junction. We

also observed a redshift of the emission wavelength as the

injection current increases for all the three devices, and this

is due to the increased junction temperature during the

testing.39

The optical output power and EQE were measured and

are presented in Fig. 7. Correspondingly, we observed the

lowest optical output power and EQE from the Reference

Device. Because of the improved current spreading effect,

the optical output power in Device A is increased by 8.46%

and 9.34% at 20 and 200 mA, respectively, as compared to

the Reference Device. For Device B, an enhancement of

18.08% and 20.87% for the optical output power is realized

at 20 and 200 mA, respectively, when compared to the

Reference Device.

In conclusion, the InGaN/GaN LED with a pþ-GaN/

InGaN/nþ-GaN polarization tunnel junction has been pro-

posed and studied in this work. Our findings indicate that the

polarization induced electric field in the pþ-GaN/InGaN/

nþ-GaN polarization tunnel junction further increases the

magnitude of the total electric field, and thus enhances the

electrical performance for the proposed device when com-

pared to the InGaN/GaN LED using the conventional pþ/

nþ-GaN tunnel junction. Moreover, the increased magnitude

of the electric field within the tunnel junction also increases

the carrier tunneling probability, and promotes the carrier

injection into the MQWs and therefore enhances the optical

output power and EQE for the proposed device.

This work was supported by the National Research

Foundation of Singapore under Grant Nos. NRF-CRP-6-

2010-2 and NRF-RF-2009-09 and the Singapore Agency for

Science, Technology and Research (A*STAR) SERC under

Grant No. 112 120 2009. The authors would like to thank Ke

Zheng for the deposition of indium tin oxide (ITO).

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effect of InGaN thickness and InN fraction on the electrical property of

the InGaN/GaN LED with pþ-GaN/InGaN/nþ-GaN polarization tunnel

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